[Haskell-cafe] Problems with type family in a class

[Niklas Haas]

On Mon, 21 Jul 2014 02:16:46 -0400, Leza Morais Lutonda <leza.ml at fecrd.cujae.edu.cu> wrote:
> Yes, it works as expected, e.g, now I have instances of 
> FourierTrasformable for List [Double] and for HMatrix.Vector Double, and 
> the following works well:
> 
> > fft (toComplex [1,2,3,4 :: Double])
> [10.0 :+ 0.0,(-2.0) :+ 2.0,(-2.0) :+ 0.0,(-2.0) :+ (-2.0)]
> > fft (toComplex $ vector [1,2,3,4])
> fromList [10.0 :+ 0.0,(-2.0) :+ 2.0,(-2.0) :+ 0.0,(-2.0) :+ (-2.0)]
> 
> 
> I think (if I understand well) this is because the constraints 
> (RealSignalType (ComplexSignalType s) ~ s) in the Complexable class 
> definition and (csx ~ ComplexSignalType s, ComplexSignal csx) in the fft 
> definition makes a one-by-one dependency between s and csx.

I think you may be right. I missed the changes to the ComplexSignal
class in between this and your original edit, I was just fixated on the
fact that your new ‘fft’ definition looked nearly identical to your
first.

You can drop the forall csx. bit, for what it's worth, since csx is an
unbound variable either way (and hence implicitly quantified).